Various microbiological, chemical and physical analyses are conducted on fermented milks in accordance with the standard offi cial procedures of countries, to ensure quality and safety of fi nal products. In addition, sensory evaluations allow guaranteeing that the product meets the required standards defi ned by the dairy company.
For microbiological analyses of fermented milks, counts of specifi c microfl ora are realized at the end of production. Target values of counts depend on countries regulations, health claims and specifi cation given by the producer. As an example, for yogurt, total counts of Lb. delbrueckii subsp.
bulgaricus and S. thermophilus must be higher than 107 CFU/g at shelf life.
The presence of spoilage microfl ora such as coliforms, yeasts or molds, and pathogenic bacteria including Listeria monocytogenes and Salmonella subsp. has to be checked. Yeast and molds have to be detected in yogurts samples, especially in fruits yogurts that have been incubated at 30°C or 25°C during 3 or 10 days. As an example, the target values that are used in France correspond to the absence of L. monocytogenes and Salmonella subsp. in 25 g of product, less than 10 CFU/g of Enterobacteriaceae and total coliforms and less than 100 CFU/g of yeasts and molds. The frequency of sampling is defi ned by each dairy plant, according to its own good hygiene practices.
By considering chemical analyses, the fat and total solid content are controlled in the fermented milks. Acidity of the products is measured, either from pH and/or titratable acidity.
Physical characteristics of yogurt such as the lack of visual whey separation and the viscosity are important parameters of the quality and of consumers’ acceptance. They are generally determined one day after packaging. Set yogurts, stirred yogurts and drinking yogurts have different physical properties that must be checked. As yogurt can be either a viscoelastic fl uid (stirred or drinking yogurt) or a viscoelastic solid (set
yogurt), different texture parameters such as fi rmness, consistency and viscosity are estimated, by using specifi c instruments such as penetrometer, viscometer or rheometer (Chammas et al. 2006).
Sensory evaluations are performed with panelists experienced in the evaluation of fermented milks in order to describe the sensory characteristics of the products. Appearance (color, syneresis, smooth), fl avor (odor, aroma), taste and aftertaste (acid, fresh, persistency) and texture (palatability, fi rmness, consistency, fl uidifying) of yogurt samples are evaluated in order to verify the consumer acceptability (Martin et al. 1999, Soukoulis et al. 2007). An example of sensory attributes used to describe the sensory characteristics of yogurts is showed in Table 3.
Table 3. Defi nitions of the descriptors (sensory attributes) used for the evaluation of the yogurts. Adapted from Soukoulis et al. (2007).
Attribute Description
Color Color of the product (white, whitish, yellowish)
Syneresis Visual observation of the yogurt surface; Level of whey drainage after inserting the spoon into the curd
Odor Intensity of acetaldehyde immediately after removing the lid Aroma Flavor defect (e.g., unclean, masked, unnatural, cooked, lacks
freshness) by smelling and oral perception of samples
Palatability Taste of samples considering several attributes associated with taste (e.g., unclean, unnatural, whey, refreshing perception) and aftertaste (e.g., sourness, astringency, sweetness, bitterness, saltiness) Firmness Hardness, brittleness, gumminess, gelatin-like texture of the
coagulum
Consistency Viscosity when stirring the samples with the spoon; rheological behavior of yogurt in the mouth
Overall acceptance Overall score of samples considering the appearance, taste, texture and fl avor profi les
6 Conclusion
Whatever the considered fermented milk, consumers are looking for products with elevated shelf life and stability, good consistency or viscosity, pleasant taste and mouth feel, together with high and reliable quality. In addition, other factors such as usefulness (drinkable or spoonable, on the go, different sizes), nutritional and functional benefits (probiotic microorganisms, bioactive peptides, low-fat products), ethnical constraints (Halal, Kosher), variety of the products (traditional, regional, fun) are often considered. More recently, investigations look for its availability to everybody (in the context of food security) and the consideration of environmental factors (energy and water sobriety, sustainable development).
These requirements, together with an increasing demand, allow developing and modifying the yogurt industry to reach high quality standards, by considering different criteria:
• Food quality: Consistency of product, sensory properties, critical quality assurance in production;
• Food safety: Microbiological safety, chemical contamination, traceability;
• Environment: GHG emissions, carbon footprint, packages waste, energy and water saving, life-cycle assessments;
• Supply capability: supply fl exibility, delivery without defects and with high accuracy;
• Operational effi ciency and costs: Overall equipment effectiveness and time utilization, total effective equipment performance, product losses and yields, media consumption, staff time utilization, operating and investment costs.
Even if these developments principally concern the yogurt industry, the industrial manufacture of other fermented milks in the world is also concerned with the aim to increase their level of quality.
Finally, some changes regularly occur concerning the market of the fermented milks. The most recent one concerns the very quick development of the Greek type yogurt chain, as a result of consumer’s demand, that raises the global demand for fermented milks in the USA. In addition, yogurt consumption has strongly expanded in Asia, due to increasing purchasing power and spending patterns.
Keywords: Fermented milk, Industrial process, Lactic acid bacteria,
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